Waves carry energy
A wave is a disturbance that carries energy from one place to another. When you drop a stone in a pond, ripples spread out, but the water doesn’t travel across the pond. It just bobs up and down. Waves move energy, not matter.
Two kinds of waves
| Transverse waves | Longitudinal waves |
|---|---|
| Particles move up and down, at right angles to the wave’s direction | Particles move back and forth, in the same direction as the wave |
| Have crests (tops) and troughs (bottoms) | Have compressions (squeezed) and rarefactions (spread out) |
| Waves on a rope, light, ocean ripples | Sound, a slinky pushed and pulled |
Parts of a wave
| Part | Meaning |
|---|---|
| Wavelength | Distance from one crest (or compression) to the next |
| Amplitude | How far particles move from rest; bigger amplitude = more energy |
| Frequency | How many waves pass each second, in hertz (Hz) |
| Speed | How fast the wave travels |
Wave speed = frequency × wavelength
What is sound?
Sound is made when something vibrates. A vibration pushes the air particles next to it, which push the next ones, and so on. This creates a sound wave, a longitudinal wave of squeezed and spread-out air, that travels to your ear.
- Pluck a guitar string → the string vibrates.
- Speak → your vocal cords vibrate.
- Hit a drum → the drumhead vibrates.
Sound needs a medium
Sound must travel through a material, called a medium. It can’t travel through empty space, so space is silent.
| Medium | Speed of sound |
|---|---|
| Air | About 343 m/s |
| Water | About 1480 m/s |
| Steel | About 5900 m/s |
Sound travels faster in solids because the particles are closer together. That’s why you can hear a train coming by putting your ear to the rail (never actually do this!).
Light travels about a million times faster than sound. That’s why you see lightning before you hear thunder. Count the seconds between them and divide by 3 to estimate how many kilometres away the storm is.
Pitch and frequency
Pitch is how high or low a sound seems.
- High frequency = high pitch (a whistle, a bird)
- Low frequency = low pitch (a tuba, thunder)
Humans hear about 20 Hz to 20 000 Hz.
| Animal | Hearing range |
|---|---|
| Humans | 20–20 000 Hz |
| Dogs | About 40–45 000 Hz |
| Bats | Up to about 200 000 Hz |
| Elephants | As low as about 14 Hz |
Sounds above human hearing are called ultrasound. Sounds below are called infrasound.
Loudness and amplitude
Loudness depends on amplitude. A bigger vibration carries more energy and sounds louder. Loudness is measured in decibels (dB).
| Sound | Decibels |
|---|---|
| Breathing | 10 |
| Whisper | 30 |
| Conversation | 60 |
| Busy traffic | 85 |
| Rock concert | 110 |
| Jet engine nearby | 140 |
Long exposure to sounds above 85 dB can damage your hearing permanently. Wear ear protection at loud events.
Echoes and sound behaviour
- An echo is sound reflecting back from a hard surface, like a cliff.
- Soft materials like curtains and carpets absorb sound, making rooms quieter.
- Sound bends around corners, which is why you can hear someone in the next room.
Echolocation
Bats and dolphins make high-pitched sounds and listen for echoes to find food and obstacles. Ships use the same idea in sonar to measure ocean depth and find fish. Doctors use ultrasound echoes to see babies before birth.
How you hear
- Sound waves enter your ear canal.
- They make your eardrum vibrate.
- Three tiny bones pass the vibrations along.
- The cochlea turns the vibrations into nerve signals.
- Your brain recognizes the sound.
Common mistakes
- Thinking sound can travel through space.
- Mixing up pitch and loudness. Pitch is frequency; loudness is amplitude.
- Thinking waves carry water or air along with them.
Try it yourself
- Stretch a slinky along the floor. Make a transverse wave and a longitudinal wave.
- Make a string telephone with two cups. Does sound travel better when the string is tight or loose?
- Fill glasses with different amounts of water and tap them. Which makes the highest pitch?
Quick recap
- Waves carry energy, not matter.
- Transverse waves move up and down; longitudinal waves move back and forth.
- Sound is a longitudinal wave made by vibrations and needs a medium.
- Frequency decides pitch; amplitude decides loudness.
- Echoes are reflected sound, used in echolocation and sonar.